1. Field of the Invention
This invention relates to stringed musical instruments, and more particularly to an improved bridge for use with such instruments.
2. The Prior Art
In stringed musical instruments such as guitars, violins, pianos, and the like, sound is produced by causing one or more tightly stretched strings to vibrate, the frequency at which the string vibrates, and thus the resultant sound output, being dependent on a number of factors including the string length, tension, and string caliper (thickness.) Vibrations of the strings are coupled through a bridge to a soundboard, and through the soundboard to a sound cavity. The strength or intensity of the sound obtained from the instrument is dependent to a large extent on the amplitude of the soundboard vibration. The quality or harmonic spectrum of the sound obtained from the string instrument is dependent to a large extent on the efficiency of driving of the normal modes of the soundboard at each characteristic frequency defined by the soundboard structure.
An important factor in determining the amplitude of soundboard vibration, and thus the response of the instrument at various frequencies, is the efficiency of the coupling between the bridge and soundboard of the instrument at these frequencies. The efficiency of this coupling is governed by mechanical impedance, mechanical impedance being defined formally as the complex ratio of the oscillatory driving force applied by the bridge to the soundboard at a given point to the resulting velocity experienced by the soundboard at the point. Mechanical compliance is essentially the reciprocal of mechanical impedance. Thus, if the bridge of a musical instrument is to transmit effectively a significant vibrational amplitude to the soundboard, the mechanical compliance between the bridge and the soundboard must be high (the mechanical impedance must be low.)
However, it has been found that mechanical impedance is frequency dependent, increasing with frequency. Thus, for effective driving of a soundboard over the many octave range of a musical instrument, the mechanical impedance between the bridge and soundboard has to be frequency adjusted for optimum driving, the bridge being designed so as to be capable of large amplitude low-frequency motion on its bass end and lower amplitude higher-frequency motion on its treble end. The reason for the frequency dependence of the mechanical impedance is that more energy is required to drive a given mass at a higher frequency than at a lower frequency and thus, for a symetrical bridge, the mechanical impedance increases as the frequency increases.
From the above, it is apparent that to minimize impedance, a bridge having minimum mass should be utilized. This is most easily accomplished by utilizing a narrow bridge. However, the bridge must be wide enough to couple the driving force effectively to the soundboard (i.e. to drive a sufficient surface area of the zone of the soundboard to get the fundamental mode driven effectively at respective frequencies.) At low frequencies, a fairly large zone must be driven, and therefore the bridge must be wider, while at higher frequencies the zone being driven is relatively small, and therefore a relatively narrow bridge can be utilized. Thus, fortuitously, the lower impedance at low frequencies permits the use of the wider bridge in the low frequency region which is required to effectively drive the large zone being driven at low frequencies without resulting in unacceptable mechanical impedance levels, while, in the higher frequency regions, where mechanical impedance is greater, a narrow bridge may be utilized since only a small zone of the soundboard is favorable driven at the higher frequencies for a soundboard correspondingly structured to be frequency-dependent. The width of the bridge in the low frequency region is limited by the fact that if the bridge is too large, unacceptable mechanical impedance levels will result and the bridge will serve to stiffen the soundboard, preventing it from vibrating effectively, while if the bridge is too narrow in this region, it will not couple effectively. At the high-frequency end, the bridge should be made as narrow as possible without impairing its ability to couple effectively or adding undue mechanical strain to the soundboard.
From the above, it is apparent that the symetrical bridges of uniform width (and other mechanical parameters) which have heretofore been utilized on most stringed musical instruments have a significantly higher mechanical impedance at their treble end than at their bass end and thus have a nonuniform frequency response, being particularly weak in the treble register. Further, in order to achieve a reasonable treble response, the width of these bridges is not normally sufficient to effectively couple at the bass end of the bridge resulting in a corresponding degradation in the bass response as well. These instruments thus provide an uneven frequency response which is significantly below optimal at all frequencies.
U.S. Pat. No. 3,443,465 titled "Guitar Construction" issued to this inventor on May 13, 1969 does teach the use of a somewhat asymmetrical bridge. However, this patent is primarily concerned with providing a bridge with separate bass and treble regions which are decoupled from each other and, while this patent does show a bridge which is wider at its bass end that at its treble end, it does not disclose the specific structures shown and claimed herein.